The present invention relates to a printer in which paper is fed and discharged after printing by the flexibility of the paper and no elements of the printer engage or touch the paper after the thermal ribbon is separated from the paper, such as a thermal printer, which can print on thick paper, etc.
In conventional printers, for example a thermal printer, it feeds print ribbon 2 wound on supply core 1, puts the print ribbon 2 on paper 3, and prints on paper 3 with print head 5 by using the print ribbon 2 when the paper 3 and the print ribbon 2 put together pass between platen 4 and the print head 5, as shown in FIG. 1. After that, print ribbon 2 is pulled off paper 3 and then wound onto take-up core 6.
In this kind of printer, to detect the presence of print ribbon 2, a transmission-type sensor 7, for example, is conventionally provided. Sensor 7 is used to detect the end of print ribbon 2 when the end 2a of print ribbon 2 is separated from supply core 1 and passes through the position of the sensor 7, as shown in FIG. 2.
However, it is not possible to detect the end of ribbon with such a ribbon end detecting mechanism when print ribbon 2 having such an end 2a firmly fastened to core 1 is used, because the end 2a is not separated from supply core 1 and does not pass through the position of sensor 7 even if print ribbon 2 runs out. Therefore, even if print ribbon 2 runs out, the printing operation is not terminated and continues successively by using the same portion of print ribbon 2. For example, in a thermal printer, there are problems in which the base of print ribbon 2 melts by the heat from print head 5 and it sticks to print head 5. Therefore, it is not possible to use print ribbon 2 having an end 2a firmly fastened to core 1 in the conventional printers.
In this kind of printer, drum gear 9 is conventionally engaged with friction gear 8 which is fixed to the same shaft as supply core 1 to prevent slack and wrinkles of print ribbon 2 by applying back tension, and friction member 13 which is held with holder 11 and biases with friction spring 12 is pushed against the circumference of friction drum 10 of the same shaft as drum gear 9 to apply a rotational load to supply core 1.
However, in such a back tension mechanism, the back tension which is applied to print ribbon 2 by the ribbon feed force increases when the diameter of the ribbon wound on supply core 1 becomes small, as shown in FIG. 4, although the rotational load which is applied to supply core 1 is constant. Therefore, when the rotational load is set at a large value and the diameter of a ribbon becomes small, the ribbon is not fed properly and slips, lowering the print quality, and in some cases print ribbon 2 is broken. On the contrary, when the rotational load is set at a small value and the diameter of a ribbon is large, as shown in FIG. 3, the back tension becomes insufficient, causing generation of slack and wrinkles in print ribbon 2.
Further, as shown in FIG. 5, print ribbon 2 is conventionally pulled off the printed paper 3 by release plate 14 being attached to thermal head 5, or by thermal head 5 itself without release plate 14 being attached to thermal head 5, which is different from the illustrated example of the figure, and wound onto the take-up core, which is not illustrated.
However, in such a ribbon release mechanism, the ribbon take-up force and the push-up force provided by the flexibility of paper 3, which straightens up itself, are applied to thermal head 5, and reduce the pushing force of thermal head 5 against platen 4. In case that, especially when a paper ribbon or a thick paper, such as tag paper, is used, there are problems that lower the print quality because clear transfer is not performed and because blurring occurs.
A first object of this invention is therefore to provide a printer which enables the use of not only a print ribbon without one end fastened to the supply core, but also a print ribbon having one end firmly fastened to the supply core.
A second object of this invention is to provide a printer which gives good back tension to roll materials, such as print ribbon wound on the supply core, all the time regardless of the size of the diameter of roll materials.
A third object of this invention is to provide a printer in which paper is fed and discharged after printing by its own flexibility and which prevents the lowering of print quality by not applying any external force, which reduces the pushing force, to the thermal head.